1. Field of the Invention
The present invention relates to a monolithic ceramic electronic component and a method for manufacturing the same. In particular, the present invention relates to a monolithic ceramic electronic component having a structure in which internal electrodes are disposed between ceramic layers such that a portion of each internal electrode is led to an end surface and an external electrode is disposed on an end surface of a ceramic laminate so as to connect to the internal electrode layers, as well as a method for manufacturing the monolithic ceramic electronic component.
2. Description of the Related Art
In general, a surface mount monolithic ceramic capacitor that is a typical monolithic ceramic electronic component has a structure in which, as shown in FIG. 15, a plurality of internal electrode layers 52 (52a, 52b) are disposed opposite to each other with ceramic layers 53 therebetween while being alternately led to opposite end surfaces 54 (54a, 54b) so as to constitute a ceramic laminate 51, and a pair of external electrodes 55 (55a, 55b) are disposed on the two end portions of the ceramic laminate 51 so as to connect to the internal electrode layers 52 (52a, 52b).
In the case where the monolithic ceramic electronic component provided with the external electrodes (in this example, the monolithic ceramic capacitor) is produced, in general, the external electrodes are formed by the procedure as described below and in Japanese Unexamined Patent Application Publication No. 2003-331649.
(1) As shown in FIG. 16A, an electrically conductive paste 62 having a predetermined thickness is applied to a planar table 61.
(2) As shown in FIG. 16B, an end surface 54 of the ceramic laminate 51 is immersed into the electrically conductive paste 62.
(3) As shown in FIG. 16C, the ceramic laminate 51 is pulled up from the electrically conductive paste 62. In this manner, a thick film (electrically conductive paste film) 63 serving as the external electrode 55 (FIG. 15) is formed on the end surface 54 of the ceramic laminate 51.
(4) The ceramic laminate 51 provided with the thick film (electrically conductive paste film) 63 is fired so as to form the external electrode 55.
In general, the thus formed external electrode is subjected to a plating treatment in order to improve the solder leaching resistance, solderability, and the like, so that a Ni plating film, a Sn plating film, or the like is formed on the surface of the external electrode.
It is desirable that the external electrode formed as described above has a uniform thickness on the whole from the viewpoint of the weather resistance, the reliability in the connection to the internal electrode layers, the shape and dimension stability of a product, and the like. However, in the stage of the item (3) in which the ceramic laminate 51 is pulled up from the electrically conductive paste 62, the electrically conductive paste 62 adhered to the end surface 54 of the ceramic laminate 51 is drawn toward a center region X of the end surface 54 of the ceramic laminate 51 in relation to the self weight, the stress generated during the pulling up, and the like. Consequently, as shown in FIG. 16C, the thickness of the external electrode 55 in the perimeter region Y of the end surface 54 is decreased as compared with the thickness of the center region X.
As a result, when the external electrode is subjected to Ni plating or Sn plating, a plating solution is liable to intrude into the ceramic laminate and, thereby, deterioration of the insulation resistance and deterioration of the weather resistance occur easily.
In general, metal components, e.g., Ni and Cu, constituting the internal electrode layer has a sintering temperature lower than that of the surrounding ceramic layer, and the internal electrode layer begins to shrink earlier during the firing. Therefore, as schematically shown in FIG. 17B, pores 65 are easily generated at end portions of the internal electrode layers 52. In the case where such a pore 65 is generated, the influence of intrusion of the plating solution is further increased.
In order to solve the problems resulting from the intrusion of the plating solution, a method has been proposed, in which glass is added to the electrically conductive paste for forming an external electrode, a ceramic laminate is sealed by a glass film formed in the vicinity of the interface to the ceramic laminate during firing and, thereby, intrusion of the plating solution is suppressed and prevented.
However, in order to completely prevent the intrusion of the plating solution, it is necessary to add large amounts of glass to the paste or set the baking temperature of the electrically conductive paste at a high level. If the amount of addition of the glass to the electrically conductive paste for forming the external electrode is increased, there are problems in that, for example, poor plating occurs as a result of rising (excessive deposition) of glass to the external electrode surface.